Directly treating sleep in the context of PTSD and MDD is feasible and can lead to robust improvements in sleep, though whether improving sleep can enhance PTSD and MDD outcomes remains to be established. This study will enroll and randomize 150 participants with PTSD, MDD and insomnia. Following baseline assessments (T1) participants will be randomized to receive cognitive-behavioral therapy for insomnia(CBTi), a well-supported and highly effective insomnia treatment, or to a monitor only control condition. Following this first intervention period all participants will receive cognitive processing therapy, a trauma focused therapy with known effects on PTSD and depression. The study will test whether and how CBTi may(1) achieve improvements in PTSD and MDD symptom severity and (2) lead to enhanced response to subsequent treatment with cognitive processing therapy.